1. Field of the Invention
The present invention relates to mobile radio communication and, in particular, to personal stations and cell stations which allow for multicast communication between a personal station or a cell station and a number of personal stations.
2. Related Art
There have been great advancements in information technologies in recent years, with much attention being focused on mobile communication systems of superior performance and responsiveness, which have been put to a variety of uses such as portable phones and cordless phones. However, the benefits due to the realization of mobile communication systems have been outstripped by the increased demands on radio communication systems. Increasing use is also being made of mobile communication systems using digital methods such as TDMA/TDD techniques in order to make efficient use of channels and improve communication services. For mobile communication systems using TDMA/TDD techniques, a frequency (one carrier) is divided into time slots of a predetermined time period. In order to achieve full duplex communication, an equal number of up and down time slots are provided as TDMA frames, with one up and one down time slot forming one radio channel.
In order to achieve full-duplex communication of audio data or non-audio data, between a cell station and one personal station, or (directly) between one person station and another, one radio channel is used. When communication between one personal station and another is achieved via a cell station, then the use of two radio channels becomes necessary. Such radio channels can be divided in terms of function into control channels and communication channels. Control channels are used for the sending and receiving of control signals such as those for positional registration or for outgoing/incoming control signals, while communication channels are principally used for the sending and receiving of information signals such as audio information or data. A number of separate carriers are allocated to the control channels and communications channels, respectively. Personal stations are set up so as to always observe the control channels, while the cell station manages the personal stations by using these control channels. Once personal stations transfer to a communications channel indicated on a control channel, the transmission of audio and non-audio data can be commenced.
There are also the following techniques described below for multicast communication, that is to say, the communication of information of a same content to a number of personal stations.
The first example of a multicast communication technique is taught by U.S. Pat. No. 5,081,704. Under this technique, the cell station first divides the personal stations to receive the signal transmitted using a notification information signal into groups by means of the control channel, before setting up the control channel for sending and receiving signals between the cell station and the personal stations for multicast communication use, and then executing communication with each of the personal stations in the group successively in order.
The second example of a multicast communication technique is taught by Japanese Patent 63-282129. Under this technique, the cell station, on receiving a request for multicast transmission from a personal station, transmits a multicast communication signal to all of the personal stations in the group. Here, "all of the personal stations in the group" refers to all of the personal stations which are positionally registered as being in the radio cell covered by the cell station in question, and which are capable of mobile communication, with each personal station being allocated its own personal station number for identification purposes.
There have, however, been the following problems with the techniques described above.
The first multicast communication technique involves the dividing of all the personal stations to receive the signal into groups and the specifying of a control channel on which each personal station in a specified group observes an individual call signal directed towards itself, so that it is not possible to call all of the personal stations belonging to a group concurrently. Therefore, by using the control channel for the transmission and reception of control signals between the cell station and the personal stations when there is positional registration or outgoing/incoming control signals and suchlike, since the notification information is multicast transmitted to all of the personal stations in the radio cell, then compared with a communications channel, there is the problem that only small amount of data can be transmitted in one time unit.
Since under the second multicast communication technique multicast communication must be executed via the cell station, then multicast communication will not be possible when there are no communication channels open for use by the cell station, for example, due to the communication channels being used for communication between use by the cell station and other personal stations. Therefore, due to reasons such as the positioning of a personal station in a given cell at a point where it is also included in an overlapping area of a neighboring cell, when the signal for such a personal station suffers from electric wave interference due to the stations in the neighboring radio cell, there is the problem of the cell station for said personal station not being able to actually receive the multicast communication signal on the communication channel indicated for multicast communication. Additionally, when, for reasons such as a personal station to be reached by a simultaneous call being switched OFF, or already communicating with another station, there are the problems in that the communication channel to be used for multicast communication cannot be established when multicast communication is to be commenced, and that it is not possible for a station to participate in the multicast communication once it has begun.